1. Field of the Invention
The present invention relates to a sheet processing apparatus for performing process on sheets and, in particular, a sheet processing apparatus for performing process on sheets with substantially no slack generated therein, and to an image forming apparatus whose apparatus main body is equipped with this sheet processing apparatus.
2. Related Background Art
For example, a conventional sheet processing apparatus staples sheets stacked together. As disclosed, for example, in Japanese Patent Laid-Open No. 11-322171, such a sheet processing apparatus may be provided in the main body of an image forming apparatus as a component of the image forming apparatus. Examples of the image forming apparatus include a copying machine, a printing apparatus, a laser beam printer, and a composite apparatus composed of these apparatuses.
FIG. 15 shows a conventional sheet processing apparatus 901, which performs alignment process for aligning an end portion of a sheet stack, a side stitch binding process for stapling an end portion of a sheet stack, a saddle stitch binding process for stapling the center or a portion near the center of a sheet stack to form a two-folded booklet, etc.
That is, the conventional sheet processing apparatus 901 successively receives sheets P with images formed on one side or both sides thereof in an apparatus main body 903 of an image forming apparatus 902, and while doing so, aligns the sheet widths (i.e., performs width alignment) to form a stack by a width alignment device 907. Thereafter, the sheet processing apparatus 901 staples an end portion of the sheet stack by an end portion stapler unit 908, and discharges the stack onto a sheet stacking portion 904 by a discharge roller pair 909. In this way, the conventional sheet processing apparatus 901 shown in FIG. 15 is capable of stapling an end portion of a sheet stack.
Further, the conventional sheet processing apparatus 901 stacks sheets successively received from the apparatus main body 903 of the image forming apparatus 902 on a steep, substantially straight saddle stitch process tray 910, and receives them by a stopper 911 to form a stack. Then, the sheet processing apparatus 901 performs width alignment on the sheets P by a width alignment device (not shown), and then staples two portions of the sheet stack near the center thereof by an intermediate portion stapler unit 912. Thereafter, the sheet processing apparatus 901 moves the stopper 911, and causes the stapled portion of the sheet stack to be opposed to the nip of a sheet folding roller pair 914 and a sheet pushing plate 913. And, the sheet processing apparatus,901 pushes the stapled portion of the sheet stack by the sheet pushing plate 913 to send the sheet stack to the nip of the sheet folding roller pair 914, and folds the sheet stack into two while nipping and conveying it by the sheet folding roller pair 914. Finally, the sheet processing apparatus 901 discharges the stack onto a sheet stacking portion 905 by a discharge roller pair 915. In this way, the conventional sheet processing apparatus 901 shown in FIG. 15 is also capable of forming a sheet stack into a twofold booklet.
Incidentally, to achieve space saving and meet various requirements of the users, there recently is a tendency for office machines to be reduced in installation area and to be increased in versatility. Similarly, sheet processing apparatuses also used as office machines also tend to be reduced in installation area and to be increased in versatility. Thus, even if a reduction in installation area and an increase in versatility are achieved through changing of the arrangement positions of the end portion stapler unit 908, the intermediate portion stapler unit 912, etc. it is necessary to align an end portion of a sheet stack when performing sheet alignment process, side stitching process, saddle stitching process, etc.
It should be noted, however, that when aligning sheets, the sheets may be swollen to generate slack when the sheets are abutted on the stopper 911. When such slack is generated, the end portion of the sheet stack becomes uneven, with the result that the alignment property deteriorates, making it impossible to accurately perform sheet alignment process, side stitch process, saddle stitch process, etc.